1. Field
This disclosure relates generally to semiconductor devices, and more specifically, to a low dropout voltage regulator.
2. Related Art
Recent years have seen tremendous advancements in the field of electronic circuits. One such advancement is in the area of providing a supply voltage used to operate electronic circuits. The supply voltage may vary due to various factors such as changes in the load of the circuit to which the voltage is being supplied, temperature variations, aging, and so forth. Variation of the supply voltage can affect the operation of the electronic circuit. Thus, a voltage regulator is used to maintain the output of the supply voltage at a predetermined value. Over the years, a few different types of regulators have been developed, such as a standard regulator, a linear drop-out (LDO) regulator, and a quasi-LDO regulator, with LDO regulators being the most widely used.
A conventional LDO regulator includes an error amplifier, a pass-transistor, a capacitor, and resistance network. The error amplifier is connected to the gate of the pass-transistor. The source of the pass-transistor is connected to a voltage supply VDD and the drain of the pass-transistor comprises the output terminal of the LDO regulator. The capacitor is typically connected to the output of LDO regulator external to the device in which the LDO is implemented. The resistance network also is connected to the output of the LDO regulator and in parallel with the capacitor. A node between the resistors is connected to an input terminal of the error amplifier and provides a scaled-down version of the output voltage to the error amplifier. The error amplifier also receives a reference voltage signal that is generated by an external voltage reference circuit. The error amplifier compares the reference voltage signal and the scaled down version of the output voltage signal to generate an error amplified signal, which is provided to the gate terminal of the pass-transistor. The error amplified signal is used to maintain the output of the LDO regulator at a predetermined voltage.
The LDO regulator generates a constant output voltage to the load (not shown) by providing the required load current. If the magnitude of the load current increases due to variations in the load, there is a corresponding drop in the magnitude of the output voltage. The drop in the output voltage leads to an increase in the magnitude of the error amplified signal generated by the error amplifier. The increase in the error amplified signal in turn increases the magnitude of the source-gate voltage of the pass-transistor, causing a corresponding increase in the magnitude of the drain current of the pass-transistor, and the increase in the drain current pulls up the output voltage. Thus, the magnitude of the output voltage signal is maintained at the predetermined voltage. The capacitor connected to the output terminal improves the transient response of the LDO regulator.
During steady-state operation, the magnitude of the output voltage signal is maintained at the predetermined value and the output capacitor is charged to the magnitude of the output voltage signal. If the current of the load circuit changes abruptly and the main regulation loop, formed by the error amplifier, pass-transistor and resistor network, may not respond quickly because it has a bandwidth limitation, the capacitor provides the extra charge required by the load. As a result, the magnitude of the output voltage decreases or increases from the predetermined voltage value.
The magnitude of output voltage drop or rise can be improved by either increasing the bandwidth/speed of the main regulation loop or by increasing the value of the capacitor. However, such changes have associated constraints such as power consumption, silicon area, and overall cost of the system. To increase the bandwidth or speed of the main regulation loop, the DC current should be increased, which results in higher power consumption of the system and also increased die area. The external capacitor increases cost while an internal capacitor cannot be made very large because that would require significant additional die area. Thus, there is a need for a circuit that improves the transient response of the LDO regulator yet avoids the above-mentioned constraints.